What Sick Travelers and Migrants Brought Home: GeoSentinel, 2012–2021

This report covers 2012 through 2021, including the first two years of the COVID-19 pandemic. It was later corrected by an erratum.

International travel reached both record highs and record lows between 2012 and 2021, a decade that also saw outbreaks of Zika, yellow fever and COVID-19. The easier and more frequent travel becomes, the faster infectious diseases can spread worldwide. Illness in returning travelers can act as an early warning of new or emerging diseases and help clinicians and public health workers identify, treat and prevent them.

In 2019 the World Tourism Organization counted a record 2.4 billion international tourist arrivals. Four studies estimated that 43% to 79% of travelers to low- and middle-income countries fell ill with a travel-related health problem. Some groups, including migrants and people traveling to visit friends and relatives (VFRs), face higher risk because they may be less aware of the risks, have less access to specialized care and pretravel advice, and trust the health care system less. Travelers can also carry diseases into new places: measles outbreaks across Europe in 2019 led to imported cases in U.S. communities with low vaccination coverage, in a country that had eliminated measles in 2000.

The GeoSentinel Network

GeoSentinel, a collaboration between CDC and the International Society of Travel Medicine (ISTM), began in 1995 with nine U.S. sites and expanded worldwide in 1996–1997. By December 2021 it had 71 sites in 29 countries on six continents, all travel and tropical medicine clinics led by specialists, plus 164 affiliate members who report unusual cases without entering data.

• Clinicians at each site record ill travelers' demographics, illnesses and travel details on a standard form entered into a secure CDC database. They choose from about 475 diagnoses and decide which cases to enter; some sites enter every case, others a convenience sample.
• Daily reports flag unusual patterns or clusters, and CDC and ISTM review them. When something stands out, the program manager asks the site for more detail.
• The network alerts clinicians and public health workers through internal notices, ProMED alerts and peer-reviewed papers, about 70 of them since 2012.

Until November 16, 2018, the database did not link the reason for travel and the country of exposure to each diagnosis, so results before and after that date are reported separately.

Who the U.S. sites saw

Worldwide, 198,120 patients were entered in 2012–2021, and 177,703 had at least one confirmed or probable travel-related diagnosis. Twenty U.S. sites reported 18,336 of them. The analysis covers the 17,389 U.S. residents seen after travel, with 25,973 travel-related diagnoses: 7,530 (43.3%) recent migrants and 9,859 (56.7%) returning nonmigrant travelers.

Migrants

The median age was 28.5, and most (89.8% of those with information) were treated as outpatients. Of 4,672 with information, 88.8% had received no pretravel health information.

Of 13,986 diagnoses, the most common were:

• vitamin D deficiency, 20.2%;
• Blastocystis, 10.9%;
• latent tuberculosis, 10.3%;
• strongyloidiasis, 6.7%;
• eosinophilia, 5.8%.

Neurocysticercosis made up 76.8% of neurologic diagnoses, and tuberculosis appeared in many forms, pulmonary, extrapulmonary, in the lymph nodes, disseminated and in the brain, though only 1% of migrants had active disease. Malaria was diagnosed in 54 migrants, mostly Plasmodium falciparum; of 26 with information, 88.5% had received no pretravel advice. No deaths were reported.

The authors link these conditions to hardship before and during migration: vitamin D deficiency to little sun exposure from covering clothing and a poor diet, and strongyloidiasis and latent tuberculosis to crowding, malnutrition, unsafe food and water, poor sanitation and limited health care. Because the United States has the world's largest migrant population, they urge clinicians to keep advocating for care for migrants, to prevent disease from reactivating and spreading, and to suspect tuberculosis in ill patients whose routine bacterial cultures find nothing.

Before November 2018, migrants' illnesses were most often acquired in Sub-Saharan Africa (22.7%), the Caribbean (21.3%), Central America (13.4%) and South East Asia (13.1%), with the Dominican Republic, Thailand and Haiti the top countries. Afterward the leading regions were Central America (27.6%), Sub-Saharan Africa (26.2%) and South East Asia (16.9%), led by El Salvador, Thailand and Honduras. Where the place of exposure was known, 89.3% of malaria cases in migrants before November 2018, and all of those after, were acquired in Sub-Saharan Africa.

Returning travelers

The median age was 37; 75.3% were born in the United States, and 89.4% were U.S. citizens. Most (90.6% of those with information) were outpatients. Of 8,967 with information, 65.6% had received no pretravel health information.

Of 11,987 diagnoses, 43.2% involved the gastrointestinal system. The most frequent diagnoses were acute diarrhea (16.9%), viral syndrome (4.9%), irritable bowel syndrome (4.1%), malaria (3.5%) and campylobacteriosis (3.1%). Among fevers, viral syndrome, malaria, dengue and chikungunya led. Bites were mostly from dogs (50.3%) and monkeys (18.3%). Four travelers died: two of severe P. falciparum malaria, one of COVID-19, and one of acute hepatitis with kidney failure.

Before November 2018, the leading regions of exposure were Central America (19.2%), Sub-Saharan Africa (17.7%) and the Caribbean (13.0%), and the top countries were Mexico, India and the Dominican Republic. Afterward, Sub-Saharan Africa (25.5%) led, then Central America (17.3%) and South East Asia (11.2%), with Mexico and India still the top countries.

Malaria and visiting family

Malaria was diagnosed in 421 returning travelers, 80.8% with P. falciparum. Most were infected in Sub-Saharan Africa (88.6% before November 2018, 95.9% after), and most were VFRs (70.3% and 57.9%). Among VFRs with malaria before November 2018, 70.2% had received no pretravel health information and 88.3% had taken no preventive malaria medicine; afterward, 83.3% had received no pretravel information, and none of the three with information had taken preventive medicine.

Studies of African VFR travelers suggest cost, difficulty speaking up in a clinic, concern about offending hosts and a low sense of risk all play a part. CDC recommends preventive medicine before and during travel for everyone going to an area where malaria is endemic, and the authors call for building trust and making care easier to reach for VFRs.

Three early warnings

• Dengue in Angola, 2013. In April–May 2013, sites in Canada, France, Germany, Israel and South Africa reported 10 dengue cases in travelers returning from Luanda. Angola had had no known outbreak since the 1980s and poor surveillance. With other imported cases in Portugal, these were among the first signs of a large outbreak; by May 31 Angola had 517 suspected cases and one death.
• Zika in Costa Rica, 2016. On January 26, 2016, the Massachusetts site saw a traveler back from Nosara with fever, rash, conjunctivitis, joint pain and headache after many mosquito bites. First diagnosed with dengue, the patient was confirmed by CDC testing to have Zika, the first Zika case reported from Costa Rica. The site alerted the whole network. By August 2017 Costa Rica had reported 1,920 cases.
• Yellow fever in Brazil, 2018. In January 2018 a site in the Netherlands reported yellow fever in a 46-year-old Dutch man who had been in São Paulo state. By March 15, four more sites had reported cases in travelers returning from Brazil, two of them fatal. The five made up half of all cases reported among international travelers to Brazil in that period. None had been vaccinated, and many had visited Ilha Grande. They were among the first cases from newly identified risk areas.

What COVID-19 exposed

Visits to U.S. sites fell sharply in 2020–2021 as travel collapsed. GeoSentinel did not catch COVID-19 early: U.S. sites first reported cases in March 2020, and cases of influenza-like illness in travelers from China were recognized as COVID-19 only afterward. The authors give three reasons. The daily reports were simple lists focused on specific diagnoses, so "viral illness" reported from China as early as December 2019 did not stand out. Testing for the new virus was not available early on. And many mild cases looked like flu, colds or allergies, so travelers treated themselves or saw their regular doctor. GeoSentinel is now building automated, real-time analytics, including machine learning, to spot outbreaks sooner. Since September 2021 it has also run research studies, starting with fever of unknown cause, brain function after malaria, mpox infection and antimalarial drug resistance.

Limits

The data describe only ill travelers who came to GeoSentinel sites, which choose what to enter and are concentrated in Europe and North America, so they cannot be used to estimate risk or rates. The United States has few large travel medicine centers, and many U.S. sites specialize in migrant medicine, so migrants may be overrepresented and their diagnoses shaped by routine screening on arrival. Changes to the data system over the decade make comparisons across periods difficult.

Sources

• Brown AB, Miller C, Hamer DH, et al. Travel-Related Diagnoses Among U.S. Nonmigrant Travelers or Migrants Presenting to U.S. GeoSentinel Sites — GeoSentinel Network, 2012–2021. MMWR Surveill Summ 72(SS-7). https://www.cdc.gov/mmwr/volumes/72/ss/ss7207a1.htm
• Erratum to the report: https://www.cdc.gov/mmwr/volumes/72/wr/mm7232a6.htm
• GeoSentinel: https://geosentinel.org/
• The report gives two figures for the share of migrants who were female, 47.3% in its summary and 47.4% in its results, so neither is used here. Its maps and figures come from the joint CDC–ISTM network and are not reproduced.